1. The Field of the Invention.
The present invention relates generally to prosthetic implants, and more particularly, but not necessarily entirely, to a prosthetic hip stem system for enhanced interdigitation between the prosthetic implant and either bone or cement for increasing the torsional stability of the prosthetic implant within the femur.
2. Description of Related Art
It is known in the art to replace the natural hip joint with an artificial hip stem replacement. Numerous artificial implants are available that can be installed to replace the natural hip joint with an artificial ball and socket combination. The medullary canal may be opened using a reamer to create a passage through the medullary canal in the upper end of the femur where a hip stem may be implanted. A stem or femoral component of an artificial implant is inserted into the reamed portion of the medullary canal in a secure, seated position. Typically, femoral implants include a neck member that extends outward and away from the stem and terminates in a spherical knob for insertion into the acetabulum of the hip in rotational contact therewith about the three major orthogonal axes.
There are two major systems to secure the femoral component of the implant within the medullary canal of the femur. The first system utilizes the natural tendencies of the bone and allows the bone to grow into porous sections of the implant without the aid of cement. The cementless system requires the removal of all cancellous bone and uses bone ingrowth to form a tight, secure fit between the implant and the bone, which maintains the implant within the bone. This system was first introduced nearly forty years ago and has become the preferred method of installation due in part to the strength of the connection between the implant and the bone.
The second system utilizes bone cement to maintain the implant within the bone. The use of cement requires the removal of bone tissue while leaving a layer of cancellous bone tissue to anchor the implant with the aid of cement. This process was used extensively during the 1980's and is still used today on a more limited basis.
Both systems may be advantageous depending upon a patient's needs. For example, recovery from an operation using the cementless system takes an average of about three months before the patient may return to any activity so that the bone may be permitted to grow into the pores of the implant, which results in a connection that has the potential to endure in the patient for a long period of time. This system is recommended for patients who lead active lives and is typically used in relatively young patients. Conversely, the cemented system results in a decrease in pain compared to the cementless system and an increase in joint mobility. However, the interface between the bone, the cement and the implant may not last as long as the cementless system. Therefore, the cemented system is typically used in less active, older patients.
It is a fairly common occurrence for femoral implants to loosen from the bone or cement over time due in part to the high stresses placed on the hip joint. Attempts have been made in the prior art to increase the efficiency of the bond between the implant and either bone or cement, such that the loosening of the implant from the bone or cement over time is decreased. One way of improving the adhesion of the stem of the implant to the bone or cement is found in U.S. Pat. No. 4,430,761 (granted Feb. 14, 1984 to Niederer et al.). Niederer et al. discloses a femoral prosthesis having a plurality of parallel grooves formed on the shank or stem of the implant to improve adhesion of the shank in a prepared bone cavity.
However, the system disclosed by Niederer et al. is disadvantageous for those situations where, for one reason or another, the implant must be removed and replaced. The location of the grooves at the distal end of the femoral prosthesis is disadvantageous because during the removal process in order to completely loosen the implant from the bone the surgeon must have adequate access to those portions of the implant where bone ingrowth has occurred. With grooves located on the distal end of the implant, the surgeon does not have adequate access to loosen that portion of the implant from the bone and the implant is, therefore, very difficult to remove.
There are many other systems known in the prior art for improving the adhesion between the implant and the bone or cement, such as that disclosed in U.S. Pat. No. 4,828,566 (granted on May 9, 1989 to Griss). This patent reference discloses a shank or stem having a recess in the proximal medial region with a U-shaped wire mesh disposed in said recess for providing an ingrowth of bone tissue and an absorption of shear micro movements between the bone and the implant. However, this system is disadvantageous because torsional forces may still be exerted on the implant, which may loosen the implant over time.
U.S. Pat. No. 3,965,490 (granted Jun. 29, 1976 to Murray et al.) discloses a femoral implant having one or more shallow teardrop-shaped depressions disposed in the flat sides of the curved proximal portion of the stem. These teardrop depressions provide extra surfaces and directional configuration, which facilitates retention within the medullary canal of the femur. However, this system is disadvantageous because there is a tendency for the implant to loosen from the cement due in part because the surface of the implant is smooth and does not provide a surface for interdigitation with the cement.
It is noteworthy that none of the prior art known to applicants provides a femoral implant having a tripartite differential porosity where the distal portion of the stem comprises the smoothest section, the proximal portion of the stem comprises a section rougher than the distal portion, and the teardrop recess comprises the roughest section of the stem and is rougher than the proximal portion. Applicants have discovered that it is advantageous for femoral implants used as part of a total hip replacement system to mimic the natural biomechanics of the hip through increasing the lateral offset, which is accomplished by increasing the length of the neck portion of the implant, which thereby increases the torsional load on the femoral implant. Applicants have further discovered that the use of differential roughness on the proximal portion, distal portion and the recessed portion of the stem opposes and resists the increased torsional load placed on the femoral implant. There is a long felt but unmet need, for a tripartite differential porosity femoral implant which has the ability to resist the increased torsional loads created by the larger lateral offset. This is accomplished by using a recessed section that may be advantageously located on both the posterior and anterior sides of the prosthesis, resulting in an increase in torsional stability in the connection between the stem and the femur. The increase in stability is due, at least in part, to the recessed section located at the posterior and anterior sides of the prosthesis, but not on the medial or lateral sides of the prosthesis, such that abrasion wear is not increased on the medial side.
The prior art is thus characterized by several disadvantages that are addressed by the present invention. The present invention minimizes, and in some aspects eliminates, the above-mentioned failures, and other problems, by utilizing the methods and structural features described herein.
The features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by the practice of the invention without undue experimentation. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims.